Hitherto, an α+β titanium alloy has been used as an aircraft member by utilizing its high specific strength. In recent years, the weight ratio of a titanium alloy to be used in aircraft members is increasing, and this alloy may become more and more important. In addition, for example, also in the consumer products field, an α+β titanium alloy which is characterized by high Young's modulus and light specific gravity thereof, may be often used for the application to a golf club face, etc.
Further, the high-strength α+β titanium alloy may be expected to find its future application in an automotive component wherein a reduction in the weight thereof is important, in a geothermal well casing requiring corrosion resistance and specific strength, and the like. In particular, the titanium alloy may be used in the form of a sheet in many cases, and therefore, the needs for high-strength α+β titanium alloy sheet may be high.
As the α+β titanium alloy, Ti-6% Al-4% V alloy (herein, “%” is mass %, in the same manner hereinafter) may be most widely used and a representative alloy, but this alloy cannot be cold-rolled because of its high strength and low ductility and is generally produced by hot sheet rolling or hot pack rolling. However, precise accuracy of the sheet thickness can hardly be achieved in the hot sheet rolling or hot pack rolling, and in such a production process, the production yield of the product is low, and it is difficult to produce a high-quality thin sheet product with a low cost.
For the purpose of solving such a problem, several α+β titanium alloys capable of producing a cold-rolled strip have been proposed.
Patent Documents 1 and 2 propose a low alloyed α+β titanium alloy containing Fe, O and N as main alloying elements. This titanium alloy is composed of Fe as a β stabilizing element and inexpensive elements O and N as α stabilizing elements in proper ranges and it shows a high strength and ductility balance. In addition, the above titanium alloy has high ductility at room temperature and therefore, it is an alloy also capable of producing a cold-rolled sheet product.
Patent Document 3 discloses a technique where Al contributing to the achievement of high strength but decreasing ductility so as to reduce the cold workability is added and, on the other hand, Si or C which is effective in increasing the strength, but does not deteriorate the cold rollability is added, to thereby enable cold rolling. Each of Patent Documents 4 to 8 discloses a technique for enhancing mechanical characteristics by adding Fe and O and controlling the crystal orientation, grain size or the like.
However, in practice, there is posed a problem that at the time of cold-rolling an α+β titanium alloy coil to a certain degree of rolling reduction or more, a crack along the sheet width direction starting from a so-called edge cracking, to cause a fracture through the width direction of the sheet (hereinafter, referred to as “sheet fracture”) in some cases.
When the sheet fracture occurs, the fractured sheet must be removed from the production line, and the production may be inhibited because of the reason, for example, that the removal thereof takes time, etc., and the production efficiency is reduced. Further, a safety problem may arise, for example, such that the sheet itself or a piece of the fractured sheet may come to fly suddenly due to the impact upon the fracturing.
Further, the sheet may significantly e deformed near a portion where the fracture has occurred in the sheet, and the portion cannot be used as a product in many cases. As a result, the production yield may be dropped, and the coil may be reduced in the unit mass, so as to further decrease the production efficiency and yield.
In addition, an alloying element is added to an alloy so as to impart high strength to the alloy, and accordingly the deformation resistance at room temperature is high, and a heavy load is required so as to decrease the sheet thickness by cold rolling. In particular, in an α+β titanium alloy, when the material for cold rolling has a hot-rolled texture where the basal plane of the titanium α phase is oriented in the direction close to the normal direction of the sheet surface (i.e., a texture called “Basal-texture”; hereinafter referred to as “B-texture”), the deformation in the sheet thickness direction becomes difficult.
In such a case, it is not easy to ensure a high reduction in sheet thickness during cold rolling (%) (={(sheet thickness before cold rolling−sheet thickness after cold rolling)/(sheet thickness before cold rolling)}·100) by one-time-cold-rolling-process, and depending on the sheet thickness of the final product, once or several times of intermediate annealing process(es) is(are) needed during the cold rolling processes. As a result, the number of cold rolling operations should be increased, so as to reduce the production efficiency.
Patent Document 9 discloses a technique where in commercially pure titanium, the grains are refined and hot rolling is started in β single phase region so as to prevent the generation of wrinkles or scratches. Patent Document 10 discloses an α+β casting titanium alloy of Ti—Fe—Al—O system for a golf club head. Patent Document 11 discloses an α+β titanium alloy of Ti—Fe—Al system.
Patent Document 12 discloses a titanium alloy for a golf club head, where the Young's modulus is controlled by a final finish heat treatment. Non-Patent Document 1 discloses a technique for forming a texture in pure titanium through heating to β region and subsequent uni-directional rolling in the α region.
However, these techniques are not intended to suppress the development of cracking in the sheet width direction in a coil during and after the cold rolling and further, to reduce the deformation resistance at the time of the cold rolling.
Accordingly, there has been demanded an α+β titanium alloy sheet having good handling property such that, in a coil, for example, a crack is less liable to be developed in the sheet width direction during and after the cold rolling, and the deformation resistance during the cold rolling is low.